Author: Peter McKenzie
Date: 23:43:09 09/07/03
Go up one level in this thread
On September 08, 2003 at 01:55:14, Russell Reagan wrote: >I think I understand the differences and between fail hard alpha-beta and fail >soft alpha-beta when described in higher level language (like English), but I'm >a bit shaky on the implementation differences between the two. > >As I understand it, this is fail hard alpha-beta (from Bruce's site): > >int AlphaBeta(int depth, int alpha, int beta) >{ > int val; > if (depth == 0) > return Evaluate(); > GenerateLegalMoves(); > while (MovesLeft()) { > MakeNextMove(); > val = -AlphaBeta(depth - 1, -beta, -alpha); > UnmakeMove(); > if (val >= beta) > return beta; > if (val > alpha) > alpha = val; > } > return alpha; >} looks reasonable > >How would this function be rewritten to perform fail soft alpha-beta? My best >guess is this (changes commented): > >int AlphaBeta (int depth, int alpha, int beta) { > int val; > if (depth == 0) > return Evaluate(); > GenerateLegalMoves(); > while (MovesLeft()) { > MakeMove(); ^^^^^^^^^^ you changed this from MakeNextMove() :-) > val = -AlphaBeta(depth - 1, -beta, -alpha); > UndoMove(); > if (val >= beta) > return val; // <-- Return val instead of beta yes, that looks correct > if (val > alpha) > alpha = val; > } > return val; // <-- Return val instead of alpha This is wrong. In this code, val just holds the value of the last move searched. I had a look at my code: I maintain another variable called bestScore which tracks the best score of the moves searched. That is what I return from Search(...). Not sure if that is classic fail soft, but it seems to work for me :-) >} > >Also, when called with an infinite window (ex. AlphaBeta(depth, -INFINITY, >INFINITY)), both the fail soft and fail hard versions should give the same >score, correct? I prefer to not think about this stuff too often, it has a tendency to make my brain implode...
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